Coverage is one of the primary requirements for cellular radio communication systems. The service coverage provided by a cellular network sets the limit on how sparsely the network can be deployed and hence has a direct impact on deployment cost.
Coverage therefore is one of the key design parameters in the Long Term Evolution, “LTE”, standard defined by the Third Generation Partnership Project, “3GPP”, Technical Specifications. One of the important services that an LTE network should be able to provide is voice service. Voice services are typically equal rate in both uplink and downlink. Coverage is generally limited by the uplink, because of the power limitations in mobile devices. Particularly with the short Transmission Time Intervals, “TTIs”, used in LTE, a User Equipment, “UE”, or other wireless device having limited power and operating at a cell border, for example, may not be able to transmit even a small Voice-over-IP, “VoIP”, packet in one TTI with sufficient energy to insure successful reception at a certain probability. Such conditions make multiple Hybrid Automatic Repeat reQuest, “HARQ”, retransmissions much more likely. Each such retransmission introduces an additional 8 ms delay. Many retransmissions lead to long delays, which can be intolerable in delay sensitive applications such as VoIP.
Rel. 8 of the LTE standard introduced “TTI bundling” to improve coverage of Voice over IP (VoIP). TTI bundling “bundles” four uplink transmission time intervals together, where succeeding transmission time intervals in the bundle are used for autonomous retransmissions. Correspondingly, a UE allocated bundle of TTIs transmits several redundancy versions (RVs) of the same transmission using the consecutively bundled TTIs. The network sends HARQ feedback for the bundled transmissions only when the last redundancy version is received.
Sending a bundle of transmissions using different redundancy versions increases the likelihood of successful reception and therefore reduces the likelihood of having to make one or more HARQ retransmissions. More particularly, bundling improves the likelihood of successful reception while effectively reducing the overhead of the transmissions because, rather than applying separate headers, the same header information is used in all TTIs. Channel coding efficiency also increases because of the longer code words.
The general time-structure of TTI bundling is described in FIG. 1, which illustrates transmission timing in terms of recurring frames 10, each divided into a like number of subframes 12, which are also referred to as TTIs. Note that FIG. 1 does not break out parallel timelines for downlink and uplink transmissions and subframes 12. In the diagram, a grant is sent from the network to a targeted UE or other wireless device running multiple synchronous HARQ processes. As holds in the LTE context, FIG. 1 assumes that there is a default mapping between respective HARQ processes and respective ones of the subframes 12.
Thus, the subframe 12 in which the grant is sent defines, according to the default mapping, which one of the HARQ processes the grant pertains to. Further, according to the default mapping, the transmission corresponding to the grant begins a defined number of subframes 12 after the grant. In this regard, one may view the subframe/frame timeline of FIG. 1 as illustrating a number of recurring subframes which are all mapped by default to respective ones of the involved HARQ processes. In other words, each HARQ process is associated with regularly scheduled subframes 12, according to the default mapping. To grant a transmission or retransmission to a particular one of the HARQ processes, the network sends the grant on one of the subframes 12 that by default are used to make grants to that particular HARQ process, and the network receives the transmissions or retransmissions from that particular HARQ process on the granted subframes 12, which are also known a priori according to the default mapping.
Thus, there is a regularly recurring association between subframes 12 or TTIs and HARQ processes, where that association repeats for each cycling of the multiple synchronous HARQ processes running between the network and the UE. In the Frequency Division Duplex, “FDD”, context, there are four HARQ processes running synchronously. Further, with TTI bundling, bundles 14 of consecutive subframes 12 are granted, again according to the default mapping between subframes and HARQ processes. According to this default mapping, a scheduling grant is sent to a UE 4 ms before the start of a corresponding TTI bundle 14. The UE then transmits one transport block over the four subframes of that TTI bundle 14, using one redundancy version per subframe. The UE then expects HARQ feedback from the network 16 ms after the initial grant. If negative feedback is received, a non-adaptive retransmission is performed by the UE 4 ms after the feedback.
While TTI bundling efficiently improves coverage for coverage-limited VoIP services, the standardized approach to TTI bundling has several drawbacks. At least some of these drawbacks are identified in the document, R1-120900, “Way Forward on Uplink Coverage Enhancement”, RAN 1#68 Dresden February 2012.
One noted drawback of TTI bundling is that the most commonly used VoIP codecs generate one new data frame every 20 ms. These data frames should be conveyed to the receiver with the lowest possible latency. A common requirement is to put a limit of 50 ms on the uplink physical layer delay. With this delay requirement and the 16 ms round trip time applicable in LTE with TTI bundling, segmentation and separate transmission of coding and overhead are needed to make full use of the UE transmit power. However, the resulting larger packet overhead and lower coding gain reduces coverage as compared to what would be achieved if the 20 ms frames could be sent in single transport blocks.